A secondary lithium ion battery generally contains at least one electrochemical battery cell that includes a negative electrode, a positive electrode, and a separator situated between the electrodes. The separator facilitates operation of the electrochemical battery cell by providing a porous and electrically-insulative physical support barrier between confronting faces of the two electrodes as is generally well understood in the art. To operate as intended, the separator is typically designed to have a porosity sufficient to contain a liquid electrolyte that can communicate lithium ions, yet remain thermally, chemically, and mechanically stable enough to separate the confronting faces of the negative and positive electrodes over the course of many discharge/charge cell cycles so that a short-circuit is prevented. The most commonly used separators today are made from a single extruded polyolefin sheet membrane or a laminate of several extruded polyolefin sheet membranes. Uniaxial or biaxial stretching is often relied upon during manufacture of the polyolefin sheet membrane(s) to promote the requisite porosity.
A conventional polyolefin sheet membrane, however, is potentially susceptible to certain performance declines when heated excessively. Exposure of the electrochemical battery cell to temperatures of 80° C. and above can cause the polyolefin sheet membrane to shrink, soften, and even melt. Such high temperatures can be attributed to charging-phase heat generation, ambient atmospheric temperature, or some other source. The physical distortion of a polyolefin sheet membrane may ultimately permit the electrochemical battery cell to short-circuit through direct electrical contact between the confronting faces of the negative and positive electrodes. Battery thermal runaway is also a possibility if the electrodes come into direct electrical contact with one another to an appreciable extent. The tendency of an extruded and stretched polyolefin sheet membrane to lose some thermal stability at temperatures exceeding 80° C. for prolonged periods is a potential concern for some lithium ion battery applications.
A separator fabricated at least in part from a sheet membrane constructed from one of several types of engineering polymers that exhibit better thermal stability than a polyolefin could potentially enhance the temperature operating window of an electrochemical battery cell and, consequently, the lithium ion battery. But the techniques often used to make a polyolefin sheet membrane generally cannot transform the various types of engineering polymers into a sheet membrane that exhibits sufficient porosity across its thickness at reasonable costs. The stretching techniques used to make a polyolefin sheet membrane have also been shown to adversely affect the dimensional stability of a sheet membrane formed from certain engineering polymer materials when exposed to elevated temperatures above 80° C. and, more noticeably, above 100° C. For example, when heated to such temperatures, a sheet membrane constructed from an engineering polymer may shrink in the direction that it was previously stretched.
A fabrication method that can reliably incorporate a robust array of polymer materials—both commodity polymers and engineering polymers—into a separator that is thermally stable and sufficiently porous within the construct of an electrochemical battery cell for a lithium ion battery is therefore needed.